It is known to use single acting cylinders and nitrogen accumulators to apply a downward, force on furrow openers. The raising and lowering of furrow openers can only be achieved by either raising the entire drill frame relative to the openers and the ground or raising a common locating bar for all the cylinders so that after reaching the limit of their extended strokes, the rams pull the furrow openers upwards as a result of the piston heads coming in contact with their respective cylinder caps. Such a system requires considerable space on the drill frame and a large capacity double acting cylinder to raise and lower the frame.
New Zealand Patent Specification No. 190,634 describes a hydraulic system for tractors using integrally mounted implements permitting the operator to control one double acting cylinder and two single acting cylinders through a single valve and lever. The cylinders involved are a single acting rock shaft cylinder, a single acting cylinder in a three point linkage, and a lift assist cylinder for lifting a frame having a trailing wheel. Such a hydraulic system acts on the frame itself and does not have any cushioning effect on the furrow openers attached to the frame. The absence of cushioning leads to a lack of depth control when individual furrow openers pass over undulations in the ground.
It is an object of this invention to alleviate at least partially these disadvantages.